Go Siege is a transformation of the ancient Chinese game of Go into a massively multiplayer online game in which hundreds of players can compete simultaneously.
GNU Chess lets most modern computers play a full game of chess. It has a plain terminal interface but supports visual interfaces such as X-Windows "xboard".
A multi-player networked clone of the classical Bubble Bobble board game. Throw bubbles at monsters and collect dozens of different bonuses before your co-players!